You need to edit the RadRichTextBoxRibbonUI and simply add 1 button at the desired place which in your case will be the RibbonTab with name Table. You need to add this RadRibbonButton:
The only thing that you should have in mind is that the command can only be executed when the caret is inside a table. In case the current position of the caret is not inside a table (or the table is not selected), it will not be clear which table the settings should be applied to. Therefore, in these cases, the CanExecute method of the command returns false and the button will be disabled.
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the Telerik team
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